There are known various types of rolling body screw device in which an endless circulation path for rolling bodies is formed by using a so-called return pipe. Such a rolling body screw device is equipped with a screw shaft having a helical rolling groove, a nut member having a helical load rolling groove opposed to the above-mentioned rolling groove and adapted to be threadedly engaged with the screw shaft through the intermediation of rolling bodies, and a return pipe attached to this nut member to form an endless circulation path for the rolling bodies.
The return pipe is equipped with a pair of leg portions to be inserted into the nut member and a communication path portion connecting the leg portions and is formed in a substantially U-shaped sectional configuration, wherein there is formed, from one leg portion to the other leg portion, a non-load passage in which rolling bodies can roll. On the other hand, the nut member has a pair of rolling body passing holes for the insertion of the leg portions of the return pipe that are formed with the central axis of the nut member therebetween, with the rolling body passing holes being open tangentially with respect to the inner peripheral surface of the nut member. Further, these rolling body passing holes are formed so as to be spaced apart from each other by a distance corresponding to several turns of the load rolling groove. When the leg portions of the return pipe are inserted into the rolling body passing holes, each leg portion protrudes slightly from the inner peripheral surface of the nut member, scooping up the rolling bodies from the rolling groove of the screw shaft into the return pipe. Thus, the rolling bodies that have been rolling under load between the rolling groove of the screw shaft and the load rolling groove of the nut member are relieved of the load and are detached from the rolling groove of the screw shaft when they reach the position where the leg portions of the return pipe protrude, and roll inside the return pipe under a non-load state to be returned to the rolling groove through a distance corresponding to several turns. That is, by attaching the return pipe to the nut member, there is formed an endless circulation path for the rolling bodies.
As an example of the return pipe, there is known one obtained by bending an iron tube into a substantially U-shape. This, however, involves high production cost, and is subject to variation in dimension when performing the bending.
Another known example of the return pipe is formed by splitting a return pipe into two along a plane including the non-load passage for the rolling bodies to obtain a pair of return pipe halves. Formed in each return pipe half is a non-load passage of a substantially semicircular sectional configuration; by causing these return pipe halves to abut each other, a non-load passage for the rolling bodies is completed.
As shown in FIG. 7, these return pipes are fixed to a nut member 101 by using a pipe hold down member 100 formed of a metal plate. The pipe hold down member 100 is fixed to the nut member 101 by screws so as to be astride the return pipe 102, whereby the return pipe 102 is fixed to the nut member 101, with a pair of leg portions 103, 103 inserted into rolling body passing holes 104, 104.
However, the inner diameter dimension of the non-load passage provided in the return pipe is set somewhat larger than the outer diameter dimension of the rolling bodies, so that when the rolling bodies actually circulate through the endless circulation path provided in the nut member, the rolling bodies repeatedly collide with the inner peripheral wall of the return pipe when rolling in the non-load passage. In the case in which the return pipe is formed through abutment of a pair of return pipe halves, the impact force when the rolling bodies collide with the inner peripheral wall of the return pipe acts as a force to push open the abutment surfaces of the return pipe halves, and, during the circulation of the rolling bodies, this force is continuously acting on the return pipe. Thus, as the operation time of the rolling body screw device is accumulated, a gap is generated between the abutment surfaces of the return pipe halves, so that there is a fear of causing a trouble of the rolling bodies to be rolled out through this gap.
In particular, in the case of a rolling body screw device for higher load and larger thrust, the rolling bodies collide with the inner peripheral wall of the return pipe so much the more vigorously, so that the gap is likely to be generated at an early stage, which makes it necessary to reliably effect the abutment of the pair of return pipe halves against the impact force due to this collision.